1. Field of the Invention
This invention relates generally to a thin film magnetic head used for recording and reproducing information in magnetic disk unit, and more particularly to a thin film magnetic head in which the crystal grain size of copper constituting at least one layer of thin film coil wound between a lower magnetic core and an upper magnetic core is made so fine that the electrical resistance thereof can be reduced, and thereby the distance of the thin film coil conductors can be reduced.
2. Description of the Related Art
The separate read/write magnetic head comprises a magneto-resistive head and an inductive head laminated on a non-magnetic substrate 10 made of alumina/titanium carbide, etc., as shown in a perspective view of FIG. 1. Since this invention relates to the construction of a thin film coil used in the inductive head, the term xe2x80x9cthin film magnetic headxe2x80x9d used in this Specification refers to a xe2x80x9cseparate read/write magnetic head,xe2x80x9d and/or the xe2x80x9cinductive headxe2x80x9d thereof. Since a vertical recording magnetic head has the substantially same structure as an in-plane recording magnetic head, the inductive recording head may include the vertical recording magnetic head having a return magnetic path, and the term xe2x80x9cthin film magnetic headxe2x80x9d may refer to the vertical recording magnetic head with a return magnetic path and/or a composite head of a magnetoresistive head and the vertical recording magnetic head.
In FIG. 1, the thin film magnetic head has an alumina layer 12, a lower shield 13, a magneto-resistive element 14, a magnetic film 15 serving as an upper shield and a lower magnetic core (hereinafter referred to as xe2x80x9clower magnetic corexe2x80x9d), and an upper magnetic core 16 on a non-magnetic substrate 10. In the figure, insulating films for insulating among the lower shield, the magneto-resistive element and the upper shield are omitted. A non-magnetic film made of alumina, etc. is provided between the lower magnetic core and the upper magnetic core, and ends of both the magnetic films facing each other via the non-magnetic thin film serve as magnetic poles of the thin film magnetic head.
An exciting coil is wound between the lower magnetic core and the upper magnetic core. A thin film coil is used as the exciting coil, and the number of turns of this coil is about 15 turns to maintain electromagnetic conversion characteristics between the magnetic cores and the coil. In order to reduce the inductance of the head by reducing the space occupied by this coil between the upper and lower magnetic cores, a thin film coil is provided in multiple layers, more commonly in two layers. To ensure insulation between the coil conductors and between the coil and the magnetic cores, non-magnetic insulating layers are provided in such a manner to surround the coils.
If the thin film coil conductors and the non-magnetic insulating layers can be made smaller and thinner, the magnetic poles provided in such a manner as to surround the coil conductors can be made smaller. This would lead to a reduction in the size of the thin film magnetic head, and accordingly reduce the inductance of the head, making it possible to record information at higher frequencies. Reducing the cross-sectional area of the thin film coil, however, would inevitably increase the resistance. It is necessary, therefore, to avoid increasing the resistance.
A thin film coil for thin film magnetic heads is normally manufactured by copper plating. An electrically conductive film is formed by sputtering an electrically conductive material, such as copper, on a non-magnetic thin film, made of alumina, etc., and an insulating resin layer, both laminated on a lower magnetic core. A photoresist film is applied to the surface of the electrically conductive film and baked at a predetermined temperature. The photoresist film, on which a photomask is positioned, is then light-exposed, developed and rinsed with water. Thus, a photoresist pattern corresponding to the thin film coil pattern is formed. Next, the thin film coil is formed by plating using a plating solution, such as copper sulfate solution. Then, the photoresist film is dissolved, and the electrically conductive film between the thin film coil conductors is removed by ion milling to complete the thin film coil. A photoresist is packed on the thin film coil in such a manner as to enclose the coil conductors, and then the photoresist is heated and cured at about 270xc2x0 C. to form an insulating resin film. Multiple layers of thin film coil are formed by repeating this process.
The thin film coil conductors thus formed are 2.5 to 4 xcexcm in width and height, with intervals between coil conductors being 2 to 4 xcexcm. Its average crystal grain size is as large as 1.1 to 1.5 xcexcm. Although the specific resistance of copper in bulk state is as small as 1.724 xcexcohm-cm, that of the thin film coil conductors is as large as over 2 xcexcohm-cm because of the large crystal grain size of copper in the thin film coil conductors. Because of the large specific resistance of copper in the thin film coil, the cross-sectional area of the thin film coil has to be increased to reduce the resistance of the coil.
The large crystal grain size of copper in the thin film coil not only increases the surface roughness of the thin film coil, but also may cause copper crystals to grow during plating even breaking through the photoresist film. The plating film also tends to penetrate between the electrically conductive film and the photoresist, leading to layer short-circuiting. To prevent such short-circuiting, the thickness of the insulating resin layers between the thin film coil conductors and between the layers of the thin film coil has to be increased. This could result in an increase in the sizes of the upper and lower magnetic cores.
It is therefore an object of this invention to provide a thin film magnetic head in which the recording current can abruptly arise and information can be written at higher frequencies because the time constant of the thin film coil can be reduced by reducing the specific resistance of the thin film coil and reducing insulating resin layers between the thin film coil conductors and between the coil layers.
In the thin film magnetic head according to this invention comprising a lower magnetic core, an upper magnetic core, and a thin film coil copper wound between them, the average crystal grain size of the thin film copper coil is not more than 0.5 xcexcm. More preferably, the average crystal grain size should be not more than 0.2 xcexcm. In the thin film magnetic head of the invention, it is more preferable that the surface roughness of the thin film copper coil is not more than 15 nm.